There are widely used incandescent light bulbs which produce light with a filament such as tungsten filament heated by flowing an electric current through it. Incandescent light bulbs show a radiation spectrum close to that of sunlight providing superior color rendering properties, and show high electric power-to-light conversion efficiency of 80% or higher. However, 90% or more of the components of the light radiated by incandescent light bulbs consists of infrared radiation components as shown in FIG. 1 (in the case of 3000K in FIG. 1). Therefore, the electric power-to-visible light conversion efficiency of incandescent light bulbs is as low as about 15 lm/W. In contrast, the electric power-to-visible light conversion efficiency of fluorescent lamps is about 90 lm/W, which is higher than that of incandescent light bulbs. Therefore, although incandescent light bulbs show superior color rendering properties, they have a problem that they impose large environmental loads.
Various proposals have been made so far as attempts for realizing higher efficiency, higher luminance and longer lifetime of incandescent light bulbs. For example, Patent documents 1 and 2 propose a configuration for realizing a higher filament temperature, in which an inert gas or halogen gas is enclosed in the inside of an electric bulb so that the evaporated filament material is halogenated and returned to the filament (halogen cycle) to obtain higher filament temperature. Such a lamp is generally called halogen lamp, and such a configuration provides the effects of increasing electric power-to-visible light conversion efficiency and prolonging filament lifetime. In this configuration, type of the gas to be enclosed and control of the pressure thereof are important for obtaining increased efficiency and prolonged filament lifetime.
Patent documents 3 to 5 disclose a configuration in which an infrared light reflection coating is applied on the surface of electric bulb glass to reflect infrared lights emitted from the filament and return them to the filament, so that the returned lights are absorbed by the filament. In this configuration, infrared lights are used for the re-heating of the filament to attain higher efficiency.
Patent documents 6 to 9 propose a configuration that a microstructure is produced on the filament itself, and infrared radiation is suppressed by the physical effects of the microstructure to increase the rate of visible light radiation.